1) You can build up a new rear wheel with a White Industries ENO eccentric rear hub for chain tension adjustment.
2) You can keep your current rear wheel, replace the cassette with a set of spacers and a wide base singlespeed cassette cog (Endless, Surly, King). Narrow cogs will gouge aluminium cassette bodies. Once the rear wheel is set up, if you're lucky, your chain will tension up nicely. If not, you can buy a half link to fine tune. Too tight and your chain will break, too loose and it will fall off. The Endless Bikes Kick Ass cogs have really tall teeth and make it much harder to drop your chain. They're $30 each.
3) Similar to 2) but use a chain tensioner instead of worrying about chain length. A derailleur with a small piece of cable to hold it in the right position will work or there are several SS conversion tensioners on the market.